refactor(chat): simplify input send button

src/app/chat/components/chat-send-button.tsx

@@ -8,38 +8,30 @@
  *   - 默认：粉色背景 + 渐变边缘
  *   - 聚焦 / 激活：完整 primaryGradient（accent → 透明）
  *
- * 图标：空输入显示 <Plus />，有内容时显示 <ArrowUp />
+ * 图标：固定显示 <ArrowUp />，按钮只负责发送消息。
  */
-import { ArrowUp, Plus } from "lucide-react";
+import { ArrowUp } from "lucide-react";
 
 import styles from "./chat-send-button.module.css";
 
 export interface ChatSendButtonProps {
   disabled: boolean;
-  hasContent: boolean;
-  isExpanded?: boolean;
   onClick: () => void;
 }
 
 export function ChatSendButton({
   disabled,
-  hasContent,
-  isExpanded = false,
   onClick,
 }: ChatSendButtonProps) {
-  const Icon = hasContent ? ArrowUp : Plus;
-  const ariaLabel = hasContent ? "Send message" : "Open chat actions";
-
   return (
     <button
       type="button"
       className={`${styles.button} ${disabled ? "" : styles.buttonActive}`}
       disabled={disabled}
       onClick={onClick}
-      aria-expanded={hasContent ? undefined : isExpanded}
-      aria-label={ariaLabel}
+      aria-label="Send message"
     >
-      <Icon className={styles.icon} size={24} aria-hidden="true" />
+      <ArrowUp className={styles.icon} size={24} aria-hidden="true" />
     </button>
   );
 }